Examples of semiconductors used for imaging devices are: CdZnTe, Si, CdTe, HgI.sub.2, InSb, GaAs, Ge, TiBr, PbI.sub.2.
A detector substrate may comprise a plurality of detector cells (e.g., pixel cells) defined by metal contacts on one side of the detector. The readout substrate can comprise a corresponding plurality of readout circuits or charge coupled device (CCD) cells. The readout substrate can be bump-bonded to the detector substrate with individual pixel cells being connected to corresponding readout circuits or CCD cells by respective conductive bumps.
Imaging devices of this type can be used for medical applications involving the exposure of a patient to ionizing radiation. Such applications require high radiation absorption characteristics for the detector substrate of the imaging device. Such high radiation absorption characteristics can be provided by materials using high Z element such as CdZnTe or CdTe.
Furthermore, various medical applications require high spatial resolution. For example, mammography requires the ability to observe microcalcifications which can be under 100 microns or even under 50 microns in size. The stringent requirements imposed on imaging devices require the use of small resolution elements (pixel cells), with a large arrays of such cells being needed to generate an image of a useful size.
An important step in the fabrication of such imaging devices is the bonding of the semiconductor substrate to the readout substrate, or more precisely, the bonding of detector cells to corresponding readout cells in a one-to-one correspondence.
A semiconductor pixel imaging device is disclosed in commonly assigned and copending U.S. patent application Ser. No. 08/454,789, the entirety of which is incorporated by reference herein. As mentioned in the previous paragraph, a significant aspect of this technology is the bonding of the semiconductor substrate to the readout substrate.
Typically, prior art hybrid imaging devices such as those described in U.S. Pat. No. 5,245,191, EP-A-0 571, 135, and EP-A-0 577 187 employ indium bumps for bump-bonding a detector substrate to a readout substrate.
Indium bumps are grown on the detector metal contacts (defining the cells) and on the readout cells using evaporation. Subsequently, the two different parts are brought together, aligned, and the corresponding bumps are merged. This is also termed flip-chip joining. This cold welding technique is achieved by heating the substrates at 70-120.degree. C. and applying mechanical pressure. For detectors comprising heat sensitive materials such as cadmium zinc telluride (CdZnTe) and cadmium telluride (CdTe) the use of indium bumps is advantageous in that the process can be carried out at a low temperature. The temperatures needed for indium bump-bonding, typically 70-120.degree. C., fall within an acceptable range for materials such as CdZnTe and CdTe.
However, during the development of imaging devices using indium bump-bonding, non-uniform detector response has been observed near the detector edges. A plausible explanation is that indium is escaping to the detector edges thus creating undesirable contact between edge metal contacts (edge pixels) and the detector edge.
The present invention seeks to mitigate the problems of the prior art.